Advances in pacing technology such as the development of atrial and dual-chamber pacing systems and innovations in surgical techniques have significantly expanded the use of pacemakers in treating cardiac patients. Pacemakers are employed today not only as a life-saving therapy for patients with complete heart block, but also as an effective means for preventing or interrupting certain recurrent atrial and ventricular tachycardias. This increased utilization of the artificial pacemakers in both temporary pacing and permanent implantation has resulted in a significant increase of paced patients in cardiac care units. Therefore, it has become increasingly important for computer algorithms that provide ECG surveillance to effectively monitor these patients.
Unfortunately, however, the complexity of pacemakers and their interaction with the electrical conduction system of the heart have made the design of algorithms for monitoring ECG signals from paced patients very difficult. Existing algorithms are either not designed to handle many types of pacing modes or have difficulty in achieving performance levels comparable to those obtainable when monitoring non-paced patients.
A typical pace pulse consists of two components, a main pulse and a repolarization pulse. The main pulse, which is used to stimulate the heart, is characterized by its narrow width, sharp rise and fall, and large variation in amplitude. The actual shape of the pulse depends on the output coupling design of the pacemaker. The repolarization pulse, sometimes referred to as a tail, is used to deplete the charge built up between the heart and the pacemaker. This is done to prevent electrode tip disintegration and to allow the pacemaker to sense the cardiac activities.
One of the difficulties in monitoring the ECG waves from a patient having a pacemaker coupled to his body is that the pace pulses can occur at any time. When they are between QRS complexes, they can be detected by a QRS detector so as to yield too high a heart rate. When they occur during a QRS complex they can cause incorrect feature measurement and template matching that can result in an error in QRS classification.
In some of the algorithms, the major spike of a pacer pulse is clipped. This ignores the presence of the repolarization pulse and causes a stair step in the QRS complex that may interfere with its classification. Furthermore, there is a difficulty in ensuring that the pulse causing this modification of the ECG signal is a pace pulse and not noise that may used as an indication of signal quality.
Another way of reducing the effect of pace pulses is to pass the ECG signal through means for increasing attenuation with frequency so as to attenuate the high frequency components of the pace pulses more than the components due to heart action that have a lower frequency, but this introduces distortion that interferes with pattern recognition and classification.